Process for preparing large-sized polymer particles

Abstract

 A process for preparing large-sized polymer particles having a particle diameter in a range of 0.1 to 500 µm by polymerization comprises dispersing a polymerizable monomer in an aqueous medium to prepare a monomer dispersion, combining the monomer dispersion with a dispersion of seed particles to make the polymerizable monomer absorbed and adsorbed on the seed particle, and polymerizing the polymerizable monomer in the presence of a polymerization initiator; in which process the number of average particle diameter of the monomer droplets in the monomer dispersion is not larger than that of the seed particles, and the number average particle diameter in a semi-stable condition (Dm) of the monomer droplets in the monomer dispersion satisfies the relationship:

(in which D is desired number average particle diameter of the final polymer).

Description

[0001] The present invention relates to a process for preparing large-sized polymer particles and more particularly to a process for preparing large-sized polymer particles having a particle diameter of about 0.1 to 500 µm by seeded polymerization.

[0002] Polymer particles, particularly monodisperse polymer particles having a particle diameter range of about 0.1 to 500 µm are required in many fields, but are generally quite difficult to prepare. Emulsion polymerization can produce monodisperse polymer particles relatively easily, but only small-sized particles having a diameter of less than 1 um. It is said that even under specific conditions only polymer particles having a diameter of 3 µm at most can be prepared by emulsion polymerization. On the other hand, suspension polymerization can produce relatively large polymer particles having a diameter of 1 to 200 um. The particle diameter distribution of these polymer particles, however, is broad; monodisperse polymer particles are difficult to prepare by suspension polymerization. Therefore, in preparing monodisperse, large-sized polymer particles, it is necessary to classify the polymer particles obtained by suspension polymerization. This method, however, has disadvantages in that the number of steps is increased, making the process complicated, and the yield is low.

[0003] Methods of preparing relatively large-sized, mono-disperse polymer particles are disclosed in Japanese Laid-Open Patent Application Nos. 97582/1979 and 126288/1979.

[0004] In the method of Japanese Laid-Open Patent Application No. 97582/1979 a chain transfer agent is added during emulsion polymerization to thereby prepare polymers having a much lower molecular weight than the usual polymer latex, and with these polymers as seed particles, a slightly water-soluble unsaturated monomer is absorbed on the seed particles and then polymerized. In this method, however, when commonly used oil-soluble or water-soluble polymerization initiators are used, problems such as aggregation and the formation of new particles arise. Thus large-sized, monodisperse polymer particles are difficult to prepare with high reliability and in high yield by the above method.

[0005] In the method of Japanese Laid-Open Patent Application No. 126288/1979, at the first stage, an organic compound having a solubility in water of less than 10 g_/l H₂0, which functions as a swelling aid, is made to absorb onto seed particles, and in the second stage, a slightly water-soluble monomer, the volume of which is about 100 times that of the seed particles, is made to absorb onto the seed particles to thereby prepare swollen particles and then the monomer is polymerized by the use of a water-soluble polymerization initiator (e.g. potassium persulphate) or oil-soluble polymerization initiator (e.g. azobisisobutyronitrile) while maintaining the swollen particle form. This method, however, has several disadvantages. For example, when an oil-soluble polymerization initiator is used, monomer droplets remaining unabsorbed on the seed particles are polymerized as such. As a result, a large amount of coagulate is formed and the yield is decreased. When a water-soluble polymerization initiator is used, even if the concentration of the emulsifier is below the critical micelle concentration, low molecular weight polymers formed by polymerization in an aqueous phase act as the emulsifiers, thereby allowing so-called soap-free emulsion polymerization to partially or wholly proceed, and thus the form of swollen particles cannot be maintained. Another problem of the above method is that polymer particles obtained are not in a spherical form but are deformed by the action of the organic compound of low water solubility as a swelling aid to be absorbed on the seed particles at the first stage.

[0006] In order to overcome the above problems, a method of swelling seed particles without the use of a swelling aid has been developed (see J.H. Jansson, M.C. Wellons & G.W. Poehlein, J. Polym. Sci., Polym. Lett. Ed., 21, 937-943 (1983). In accordance with this method, a monomer and an oil-soluble polymerization initiator are mixed and finely dispersed in an aqueous medium to prepare an aqueous dispersion, and this aqueous dispersion is added to a seed particle dispersion (latex), whereby the seed particles are swollen in a high swelling ratio. This method, however, has several disadvantages and is not sufficiently satisfactory for practical use.

[0007] For example, when styrene is used as the monomer and added in such an amount that the weight ratio of monomer to seed particles is 100:1, even if seed particles having a uniform diameter are used, only swollen particles having a diameter of about 1 to 3 µm can be obtained while maintaining the uniform diameter. If swollen particles having a diameter exceeding the above range are intended to be prepared, the particle diameter distribution is inevitably broadened; monodisperse, large-sized polymer particles having a diameter of more than 3 ILm cannot be obtained. In the case of monomers having a high water solubility, such as methyl methacrylate (MMA), swollen particles having a diameter range of about 1 to 3 µm are difficult to prepare while maintaining a uniform particle diameter. Even if uniform particles are obtained, their stability is poor; the uniformity is quickly lost and the particles become uneven in diameter. Moreover, in the case of monomers having a low water solubility, such as e-ethylhexyl acrylate, monomer droplets in the aqueous dispersion are stable and remain unabsorbed on the seed particles for a long period of time, thereby preventing the formation of swollen particles having a uniform particle diameter.

[0008] In summary, the method disclosed by J.H. Jansson et al. has disadvantages, for example, in that the preparation of large-sized, monodisperse polymer particles by seeded polymerization, only a limited range of monomers such as styrene can be used, and even with such monomers, monodisperse polymer particles having a particle diameter as large as more than 3 µm cannot be obtained.

[0009] An object of the present invention is to provide a process for preparing large-sized polymer particles by seeded polymerization. Another object of the present invention is to provide a process for preparing monodisperse polymer particles having a particle diameter as large as 0.1 to 500 µm, particularly 5 µm or more. Still another object of the present invention is to provide a process whereby monodisperse, large-sized polymer particles can be prepared regardless of the type of a monomer to be polymerized.

[0010] It has been found that the above objections may be attained by preparing a dispersion of a monomer to be polymerized in an aqueous dispersion, satisfying the requirements described hereinafter, and adding the monomer dispersion to a dispersion of seed particles to thereby cause the monomer to be absorbed or adsorbed on the seed particles.

[0011] According to the invention there is provided a process for preparing large-sized polymer particles having a particle diameter falling in a range of 0.1 to 500 µm (a) dispersing a polymerizable monomer in an aqueous medium to prepare a monomer dispersion, (b) combining the monomer dispersion with a dispersion of seed polymer particles so that the polymerizable monomer is absorbed or adsorbed on the seed polymer particles, and (c) polymerizing the polymerizable monomer in the presence of a polymerization initiator; characterized in that in the monomer dispersion prepared step (a), (i) the number average particle diameter of the monomer droplets in the dispersion is not larger than the average particle diameter of the seed polymer particles and (ii) the number average particle diameter in a semi-stable condition, of the monomer droplets satisfies as D, the following relationship:

where D is the desired particle diameter of the final polymer particles.

[0012] In the following reference will be made to the accompanying drawings in which:

Figure 1 is a graph showing the relationship between the number average particle diameter of monomer droplets and standing time;

Figure 2 is a scanning-type electron micrograph of polymer particles as obtained in Example 15;

Figure 3 is a scanning-type electron micrograph of polymer particles as obtained in Reference Example 1;

Figure 4 is a scanning-type electron micrograph of fluorine-containing polymer particles as obtained in Example 34;

Figure 5 is the infrared absorption spectrum of the above fluorine-containing polymer particles.

[0013] In accordance with the process of the invention, firtsly, a polymerizable monomer is finely dispersed in an aqueous medium to prepare a monomer dispersion or emulsion. This monomer dispersion should be prepared so as to meet the following requirements.

(1) The number average particle diameter of monomer droplets in the dispersion is not larger than the number average particle diameter of the seed particles with which the dispersion is later combined.

(2) The number average particle diameter, in a semi-stable condition, of the monomer droplets satisfies the relationship.

where D is the desired number average particle size of the final polymer particles.

[0014] The objects of the present invention may be attained only when the above requirements are satisfied.

[0015] If the number average particle diameter of the monomer droplets is larger than the number average particle diameter of the seed particles, the monomer droplets swell regardless of the seed particles, producing swollen particles having a broad particle diameter distribution, and thus the desired monodisperse polymer particles cannot be prepared.

[0016] Preferably, the maximum particle diameter of the monomer droplets is not larger than the number average particle diamter of the seed particles. The term "maximum particle diameter" as used herein means the particle diameter of the largest of normal monomer droplets excluding exceptionally and abnormally large monomer droplets as obtained by finely dispersing a polymerizable monomer in an aqueous medium. Thus the maximum particle diameter can be said to be the "substantially largest particle diameter".

[0017] The number average particle diameter and the maximum particle diameter of the monomer as used herein are the values as determined just before combining the monomer dispersion with the seed particle dispersion.

[0018] The number average particle diameter and the maximum particle diameter can be determined based on results as obtained by measuring an optical micrograph when they are not less than 0.8 µm, and when they are less than 0.8um, based on results of measurement by e.g., the dynamic light scattering method [see, for example, J. Chem, Phys., 72 (11), page 6024 (1980)] or the centrifugal settling of floating analytical method.

[0019] The term "number average particle diameter in a semi-stable scondition. Dm, of the monomer droplets" as used herein means the number average particle diameter of the monomer droplets in a semi-stable condition after preparation of the aqueous dispersion of the polymerizable monomer, i.e. in a condition such that changes in particle diameter on standing of the aqueous monomer dispersion become moderate. On standing, after preparation of the aqueous dispersion of the polymerizable monomer, as shown in Figure 1, the particle diameter of monomer droplets changes greatly initially but gradually reaches a constant value at which the monomer droplets are believed to be stabilized to a certain extent. In general, this stabilized condition is reached on allowing the aqueous monomer dispersion to stand, or stirring it moderately, for 3 hours after its preparation. Thus the term "number average particle diameter in a semi-stable condition, Dm, of the monomer droplets" as used herein means the number average particle diameter of the monomer droplets as determined after allowing the aqueous monomer dispersion to stand, or stirring it moderately, for 3 hours after its preparation.

[0020] Measurement of the number average particle diameter in the determination of Dm of the monomer droplets can be carried out in the same manner as described above.

[0021] One of the features of the present invention, therefore, resides in that the colloidal stability of the aqueous monomer dispersion is controlled so as to satisfy the relationship represented by equation (a) above, more preferably the relationship

(wherein D is as defined above).

[0022] In accordance with the process of the invention, secondly, the aqueous monomer dispersion as prepared above is combined with a dispersion of seed particles so that the polymerizable monomer is absorbed or adsorbed on the seed particles, whereupon swollen particles are obtained.

[0023] Thus the process of the present invention permits the production of swollen particles having a uniform particle diameter, large or small, regardless of the type of the polymerizable monomer, which could not be obtained by conventional techniques. Moreover, by using monodisperse seed particles, swollen particles in a monodisperse form can be obtained.

[0024] In the present invention, as described above, it is required that Dm and D satisfy the relationship of equation (a). If Dm is not larger than 0.5xD, some monomer droplets remain unabsorbed or unadsorbed on the seed particles and only swollen particles having a broad particle diameter distribution can be obtained. Upon polymerization of the polymerizable monomer in such a condition, undesirable polymer particles having a broad particle diameter distribution are formed rather than monodisperse polymer particles. On the other hand, if Dm is not smaller than 3.5xD, when the aqueous monomer dispersion and the seed particle dispersion are combined, swollen particles having a uniform particle diameter distribution are formed in a short time, but particle diameter distribution is immediately broadened because the particle diameter of the swollen particles is below the stable region. Therefore, even by polymerization of the polymerizable monomer in such a condition, no monodisperse polymer particles can be obtained.

[0025] The desired particle diameter of the final particles. D(µm). is defined by the following equation:

where

Ms = weight of seed particles used (solids, g),

Ds = particle diameter of seed particles (µm),

Mm = weight of monomer used (g),

ds = specific gravity of the seed particles,

dm = specific gravity of the polymer of the monomer, and

(X) = polymerization conversion (o to 1.0).

[0026] In practice, assuming that the polymerization conversion is 1.0, the equation (b) can be approximately rewritten as follows:

[0027] In a preferred embodiment of the present invention, therefore,

firstly the desired particle diameter of the final polymer particles, D, is fixed, and in order that the desired particle diameter D can be obtained, the particle diameter of seed particles, the amount of seed particles used, and the amount of the monomer used are determined according to the equation (c).

secondly a polymerizable monomer is finely dispersed in an aqueous medium to prepare a monomer dispersion in which the number average particle diameter of the monomer droplets of the seed particles and, moreover, the number average particle diameter in a semi-stable condition of the monomer droplets, Dm, satisfies the relationship:

and then

the above monomer dispersion is combined with a seed particle dispersion so that the polymerizable monomer is absorbed or adsorbed on the seed particles.

[0028] Control of the number average particle diameter Dm so as to satisfy equation (a) can be achieved by adding various compounds for adjusting the colloidal stability of monomer droplets to the monomer and/or a dispersion medium.

[0029] In order to more decrease the number average particle diameter Dm, it is sufficient to add oily substances to the monomer, said oily substances having a smaller water solubility than the monomer and exerting no adverse influences on polymerization. Oily substances the water solubility of which is not more than 1/100 of that of the monomer are preferred. Representative examples of such oily substances are organic solvents such as hexane, decane and petroleums; polymerization initiators such as lauroyl peroxide and octanoyl peroxide and monomers such as 2-ethylhexyl acrylate and stearyl methacrylate.

[0030] For example, Dm of a dispersion as prepared by finely dispersing MMA (solubility: 1.7 g/100 g H₂0) in water in the presence of a surface active agent is about 26 um. When n-hexane (solubility: 1.8x10^-5 g/100 g H₂0) is added in an amount of 1 wt.% based on the weight of MMA, the resulting number average particle diameter Dm is about 5 µm. Upon polymerization of MMA under such conditions, monodisperse MMA polymer particles having a particle diamter in the range 2 to 10 µm can be formed.

[0031] The solubility in water of the oily substance as defined in the present invention is the solubility in pure water at the temperature at which absorption or adsorption onto the seed particles is carried out. The amount of the oily substance dissolved can be measured by a suitable analytical method chosen from known techniques depending on the physical and chemical properties of the oily substance. Examples of such known techniques are the chemical titration method, the infrared spectral method, the ultraviolet spectral method, the absorbance measuring method, the polarographic method, the mass spectral method, the solvent extraction method, and the gas chromatographic method. When the oily substance is an organic peroxide, its solubility in water may be determined by the chemical titration method which is generally used and is of high accuracy. In this analysis, care should be taken so as to remove the influences of the objective substances emulsified or dispersed in water, for example, by filtration with a membrane filter.

[0032] The number average particle diameter in a semi-stable condition, Dm, can be easily determined by preliminary experiments. If, therefore, conditions such as the type and amount of the oily substance for obtaining the desired Dm are previously determined, the desired polymer particles can be easily prepared.

[0033] In order to more increase the number average particle diameter in a semi-stable condition, Dm, it is sufficient to add organic solvents completely miscible with water to the polymerizable monomer or its aqueous dispersion, and/or the seed particle dispersion, or to add inorganic metal salts to the aqueous monomer dispersion and/or the seed particle dispersion.

[0034] Typical examples of suitable organic solvents are methanol, ethanol, isopropanol, acetone, etrahydrofuran, and dimethylformamide (DMF). These organic solvents may be added before finely dispersing the polymerizable monomer.

[0035] Typical examples of suitable inorganic metal salts are NaCl, KC1, Na₂SO₄, M^gCl₂, CaCl₂, CaCo₂, Al₂(SO₄)₃. and NH₄OH. Of these compounds, polyvalent metal salts are preferred because they are very effective in increasing Dm even when used in a small amount.

[0036] As in the case of the oily substance as described above, the type and amount of the organic solvent or inorganic metal salt can be determined by preliminary experiments so that the desired Dm can be obtained.

[0037] In accordance with the present invention, the number average particle diameter Dm can be determined within the range of 0.1 to 500 µm, particularly 5 um, or more, and monodisperse polymer particles having a particle diameter corresponding to Dm can be prepared.

[0038] The process of the present invention will hereinafter be explained in more detail.

[0039] In the first step, a polymerizable monomer is finely dispersed in an aqeuous medium to prepare an aqueous monomer dispersion. This aqueous monomer dispersion is immediately combined with a seed particle dispersion, and then the monomer droplets and seed particles are brought into contact with each other, for example by stirring gently, to thereby cause the polymerizable monomer to be absorbed or adsorbed on the seed particles, that is to prepare swollen particles.

[0040] In preparation of the aqueous monomer dispersion, it is sufficient to finely disperse the polymerizable monomer in an aqueous medium in the presence of a small amount of a surface active agent by usual fine dispersing techniques such as by the use of a homomixer. a supersonic homogenizer or a high pressure piston pump-type homogenizer.

[0041] On standing of the above aqueous monomer dispersion, monomer droplets of small diameter disappear as a result of diffusion in water while on the other hand monomer droplets of large diameter swell by diffusion of the monomer. In the swelling process, the number average particle diameter increases rapidly at an early stage but after several hours, slowly. In principle, if the aqueous monomer dispersion is allowed to stand for an infinite period, it will separate into two layers of water and the polymerizable monomer. However, in 3 hours after preparation, the aqueous monomer dispersion can be deemed, as described above, to be in a semi-stable condition.

[0042] On mixing the aqueous monomer dispersion, just after its preparation, with the seed particle dispersion, swollen particles are formed as a result of absorption or adsorption of the polymerizable monomer on the seed particles.

[0043] Contacting of the polymerizable monomer with the seed particles is preferably carried out by gently stirring the system to such an extent that the dispersion does not separate.

[0044] The contact time between the polymerizable monomer and the seed particles is sufficient to be appropriately determined within the range of about 1 to 48 hours with the time required for the particle diameter to reach a semi-stable condition, for example 3 hours, as a standard.

[0045] If the desired'number average particle diameter D is smaller than the number average particle diameter in a semi-stable condition, Dm, the contact time may be short. On the contrary, if D is larger than Dm, it is preferred for the contact time to be relatively long.

[0046] The temperature of the system at which the polymerizable monomer is contacted with the seed particles is not critical as long as it is within a temperature range where the polymerizable monomer does not undergo polymerization (usually 0 to 60°C). For example, if the system is slowly heated to about 40°C, the contact time can be shortened.

[0047] The mixing ratio (by weight) of the polymerizable monomer to the seed particles is usually 6:1 or more, preferably 10:1 or more, and more preferably 20:1 or more. If the mixing ratio is less than 6:1, the seed particles are swollen only to a limited extent and the effects of the present invention cannot be obtained sufficiently.

[0048] There is no special limitation to the upper limit of the mixing ratio of the polymerizable monomer to the seed particles. As long as the relationship represented by equation (a) is satisfied, the seed particles can be easily swollen usually to about 1,000 to 10,000 times their original size.

[0049] In the above-prepared dispersion containing swollen particles, the polymerizable monomer is polymerized by a usual procedure in the presence of a polymerization initiator.

[0050] Polymerizable monomers which can be used in the present invention are radical polymerizable monomers, and a wide variety of such monomers, from those having a high water solubility to those having a low water solubility, can be used as long as they are not completely soluble in water.

[0051] Typical examples of monomers having a high water solubility are acrylonitrile, ethyl methacrylate, vinyl acetate, methyl methacrylate, vinyl chloride, and methyl acrylate.

[0052] Typical examples of monomers having a low water solubility are 2-ethylhexyl acrylate, stearyl acrylate, stearyl methacrylate, and 2,2,3,3-tetrafluoropropyl acrylate.

[0053] Other monomers which can be used include aromatic vinyl monomers such styrene, divinylbenzene and a-methylstyrene; ethylenically unsaturated carboxylic acid alkyl esters such as butyl acrylate and butyl methacrylate, and conjugated dienes such as butadiene and isoprene.

[0054] These monomers can be used alone or in combination.

[0055] As seed particles for use in the invention, those swelling on absorbing the polymerizable monomer are preferably used. Typical examples are particles of polymers such as polystyrene, styrene copolymers such as a styrene-butadiene copolymer, acrylate polymers, and vinyl acetate polymers. These seed particles are used in the form of a dispersion in water, such as a latex, emulsion or suspension. In addition, aqueous dispersions of polymers not having swelling properties, highly cross-linkable polymers, and inorganic materials the surface of which is made oleophilic can be used.

[0056] It is preferred that the seed particles have a uniform particle diameter so that the final polymer particles have a uniform particle diameter. The particle diameter of the seed particles can be determined appropriately depending on, e.g., the particle diameter of the final polymer particles and the intended end use of the final polymer particles. In general, polymer particles having a uniform particle diameter usually in a range of 0.1 to 0.9 µm as obtained by soap-free polymerization can be used.

[0057] Of course, polymer particles as prepared by the process of the present invention can be used as seed particles to prepare polymer particles having a much larger particle diameter.

[0058] Seed polymer particles having a uniform particle diameter can be prepared by techniques such as the method described in A.R. Goodall et al., J. Polym. Sci. Polym. Chem. Edition, Vol. 15, page 2193 (1977).

[0059] More specifically, uniform seed particles having a standard deviation falling within a range of 10% from a number average particle diameter are preferably used. Use of such uniform seed particles permits the preparation of monodisperse polymer particles.

[0060] The polymerization initiator is preferably an oil-soluble polymerization initiator. Water-soluble polymerization initiators are not preferred in that they tend to cause polymerization of monomers other than those of the swollen particles, thereby producing polymer particles having a broad particle diameter distribution.

[0061] Except when an oil-soluble polymerization initiator having high oleophilic properties is dissolved in the polymerizable monomer and used to decrease Dm, or when, in order to cause monomers not exhibiting affinity to seed particles (e.g. fluorine-containing monomers) to become absorbed on the seed particles, an oil-soluble polymerization initiator having high oleophilic properties is previously finely dispersed and absorbed or adsorbed on the seed particles, it is preferred to use oil-soluble polymerization initiators having a water solubility of 0.001 to 0.2 g/100g H₂0.

[0062] If the water solubility of the oil-soluble polymerization initiator is too low, it takes an undesirably long time for the polymerization initiator to be absorbed in the seed particles because fine droplets of the polymerization initiator have high stability. On the other hand, if the water solubility of the oil-soluble initiator is too large, fine droplets of the initiator have a seriously short life because of their instability. Therefore it is difficult to prepare an aqueous dispersion of the polymerization initiator in which the number average particle diameter of the droplets is not larger than than of the seed particles.

[0063] The water solubility of the polymerization initiator is determined in the same manner as described above in connection with the oily substance used to control _Dm.

[0064] Oil-soluble polymerization initiators having a water solubility of 0.001 to 0.2 g/100gH₂O which are preferably used in the invention include organic peroxides such as 3.5,5-trimethylhexanoyl peroxide, tert-butyl-peroxy 2-ethylhexanoate, and di-tert-butyl peroxide; and azo compounds such as azobisisobutyronitrile and azobiscyclohexanecarbonitrile.

[0065] If the oil-soluble polymerization initiator is in a solid form such as powder, it is preferred that the polymerization initiator be dissolved in an inert organic solvent such as toluene and cyclohexanone.

[0066] When using oil-soluble polymerization initiators having a water solubility of more than 0.2 g/100 g H₂0, it is necessary to add a dispersion aid in an amount of not less than 0.1 part by weight per 100 parts by weight of the initiator because such polymerization initiators are difficult to finely disperse. Dispersion aids which can be used for this purpose include inert oil-soluble substances having a water solubility of not less than 0.001 g/loog h₂0. Typical examples are n-hexane, heptane, octane, and dioctyl 1-chlorododecane- adipate.

[0067] Typical examples of oil-soluble polymerization initiators having a water solubility of not less than 0.2 g/100 g H₂0 are tert-butylperoxy acetate, tert-butylperoxy isobutylate, tert-butylperoxy pivalate, tert-butyl hydroperoxide, acetyl peroxide and isobutyryl peroxide.

[0068] In the process of the invention, the oil-soluble polymerization initiator may be first mixed with the polymerizable monomer and then finely dispersed in an aqeuous medium. In this case, however, the polymerizable monomer may sometimes undergo polymerization due to heat generated during the dispersing process. It is therefore preferred that the initiator be finely dispersed and added to the seed particle dispersion independently of the polymerizable monomer. Preferably, therefore, the polymerizable monomer is finely dispersed in an aqeuous medium that the number average particle diameter of the resulting monomer droplets is not larger than the number average particle diameter of the seed particles, thereby preparing a monomer dispersion and, separtely, the oil-soluble polymerization initiator is finely dispersed in an aqueous medium so that the number average particle diameter of the resulting droplets is not larger than the number average particle diameter of the seed particles, thereby preparing a polymerization initiator dispersion and, thereafter, the two dispersions are added to the seed particle dispersion.

[0069] More preferably, the above polymerization initiator dispersion is first combined with the seed particle dispersion so that the polymerization initiator is absorbed or adsorbed on the seed particles, and then the above monomer dispersion is combined with the resultant mixture so that the polymerizable monomer absorbed or adsorbed on the seed particles.

[0070] In a more preferred embodiment of the present invention, therefore,
the desired particle diameter of the final polymer particles, D, is fixed, and in order that the desired particle diameter D can be obtained, the particle diameter of the seed particles, the amount of seed particles used, and the amount of the monomer used are determined according to equation (c):

an oil-soluble polymerization initiator is finely dispersed in an aqueous medium so that the number average particle diameter of the resulting droplets is not larger than the number average particle diameter of the seed particles, to give a polymerization initiator dispersion;

this polymerization initiator dispersion is combined with a seed particle dispersion to thereby so that the polymerization initiator is absorbed or adsorbed on the seed particles,

a polymerizable monomer is finely dispersed in an aqueous medium so that the number average particle diameter of the resulting monomer droplets is not larger than the number average particle diameter of seed particles and, moreover, the number average particle diameter in a semi-stable condition of the monomer droplets, Dm, satisfies the equation:

(wherein D is the desired particle diameter of the final polymer particles), thereby preparing a monomer dispersion;

this monomer dispersion is combined with the seed particle dispersion so that the polymerizable monomer is absorbed or adsorbed on the seed particles: and

the polymerizable monomer is polymerized.

[0071] When a monomer mixture containing 40 to 100 wt% of one or more monomers of the formula:

(wherein R¹ is a hydrogen atom or a methyl group, and R^Z is a C₆-C₁₈ alkyl group) is used as the polymerizable monomer in the invention, there can be obtained an aqueous dispersion of fine polymer particles having good standing stability because the polymer has a specific gravity nearly equal to that of an aqueous medium.

[0072] The amount of monomer(s) of formula (I) is suitably 40 to 100 wt%, preferably 60 to 97 wt% and more preferably 70 to 95 wt%, based on the total weight of all monomers.

[0073] Representative examples of monomers of formula (I) are 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, lauryl acrylate, lauryl methacrylate, tridecyl acrylate, tridecyl methacrylate, stearyl acrylate and stearyl methacrylate.

[0074] If, in the formula (I), the number of carbon atoms in R² is less than 6, the specific gravity of the resulting polymer is greater than that of water and thus the desired aqueous polymer particle dispersion may not be prepared. On the other hand, if the number of carbon atoms in _R² is more than 18, the resulting monomers cannot be made absorbed on seed particles.

[0075] As other comonomers constituting the monomer mixture, any monomers copolymerizable with monomers of formula (I) can be used. In particular, aromatic vinyl monomers and ethylenically unsaturated carboxylic acid alkyl esters (others than those of formula I) are preferred. Representative examples of such comonomers are styrene a-methylstyrene, p-methylstyrene, divinylbenzene, butadiene, isoprene, vinylidene chloride, vinyl acetate, acrylonitrile, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate, acrylamide, methacrylamide, glycidyl acrylate, glycidyl methacrylate, N-methylolacrylamide, N-methylolmethacrylamide, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, diallyl phthalate, allyl acrylate, allyl methacrylate, acrylic acid, methacrylic acid, itaconic acid and fumaric acid. Particularly preferred monomers are styrene, divinylbenzene, methyl methacrylate and ethylene glycol dimethacrylate.

[0076] The monomers of formula (I) and other monomers copolymerizable therewith can be used alone or in combination.

[0077] If the proportion of monomer(s) of formula (I) in the monomer mixture is less than 40 wt%, the resulting aqueous polymer particle dispersion undesirably readily undergoes sedimentation; so that the desired aqueous polymer particle dispersion may not be obtained.

[0078] The composition of the monomer mixture is suitably determined so that the specific gravity of the resulting polymer is nearly equal to 1, usually 1.020 to 0.980, preferably 0.010 to 0.990. and more preferably 1.005 to 0.995.

[0079] A typical monomer composition producing a polymer having a specific gravity of 1.000 comprises 88 parts by weight of 2-ethylhexyl acrylate, 6 parts by weight of styrene, and 6 parts by weight of divinylbenzene.

[0080] Moreover, in accordance with the process of the invention, monodisperse, large-sized fluorine-containing polymer particles can be prepared using a monomer mixture containing a fluorine-containing monomer as the polymerizable monomer. In this case, to accelerate absorption or adsorption of the fluorine-containing monomer on the seed particles, it is preferred that an oleophilic substance be first absorbed on the seed particles and then, after combining a fluorine-containing monomer dispersion with the seed particle dispersion so that the monomer is absorbed or adsorbed on the seed particles, the fluorine-containing monomer is polymerized.

[0081] The fluorine-containing monomer as used herein is a radical polymerizable monomer having a fluorine content of not less than 25 wt%, preferably not less than 35 wt%.

[0082] Preferred examples of the fluorine-containing monomer are fluoroalkyl acrylates or methacrylates, such as 2,2,2-trifluor ethyl acrylate, 2,2,3,3-tetrafluoropropyl acrylate, 2,2,3,4.4,5,5-octafluoroamyl acrylate, and 1H, 1H, 2H, 2H-heptadecafluorodecyl acrylate. In addition there can be used, trifluorochloroethylene, vinylidene fluoride, ethylene trifluoride, ethylene tetrafluoride, trifluoropropylene, hexafluoropropene and hexafluoropropylene. Monomers not containing fluorine which may be used in combination with the fluorine-containing monomers are any monomers copolymerizable with the fluorine containing monomer. Typical examples of such comonomer not containing fluorine are aromatic vinyl compounds such as styrene, a-methylstyrene, p-methylstyrene, halogenated styrene, divinylbenzene and 4-vinylpyridine; vinyl esters such as vinyl acetate and vinyl propionate; unsaturated nitriles such as acrylonitrile; and ethylenically unsaturated carboxylic acid alkyl esters such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, lauryl acrylate, lauryl methacrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate, and N,N-dimethylaminoethyl methacrylate.

[0083] Conjugated diolefins such as butadiene and isoprene can also be used.

[0084] In addition, acrylamide, methacrylamide, glycidyl acrylate, glycidyl methacrylate, N-methylolacrylamide, N-methylolmethacrylamide, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, allylacrylate, allyl methacrylate, and the like can be used depending on the purpose of use. If necessary, acrylic acid, methacrylic acid, itaconic acid, fumaric acid and the like can be used in such an amount not to prevent swell polymerization.

[0085] If a polyfunctional vinyl monomer, such as divinylbenzene, ethylene glycol diacrylate or ethylene glycol dimethacrylate, is used in combination in an amount of not less than 0.5 wt.%, preferably 1 to 40% wt%, based on the total weight of all monomers, fusion of soft polymer particles is prevent and the hardness and stiffness of the polymers are increased. Thus the final polymer particles are preferably used in the field of, e.g., lubricants.

[0086] The proportion of the fluorine-containing monomer in the monomer is mixture suitably such that the fluorine content of the ultimate polymer is not less than 3 wt.%, preferably not less than 10 wt.%, and more preferably not less than 20 wt.%. If the fluorine content of the polymer is less than 3 wt.%, particle aggregation is improved only to a limited extent.

[0087] The oleophillic substance to be absorbed on the seed particles is a substance having a water solubility of preferably not more than 0.02 g/100 g H₂0, more preferably 0.001 to 0.02 g/100 g H₂0, and a molecular weight of not more than 5,000, preferably not more than 500. Typical examples of such oleophilic substance are 1-chlorododecane, hexane, dioctyl adiphate, stearyl methacrylate, and monomers which are also capable of acting as polymerization initiators, such as dioctanoyl peroxide, lauroyl peroxide, and 3,5,5-trimethylhexanoyl peroxide.

[0088] The method of causing the oleophilic substance to be absorbed on the seed particles is not critical. Usually the oleophilic substance is dispersed in water by the use of e.g., soap and added to water containing the seed particles. It is also possible to accelerate the transfer of the oleophilic substance to the seed particles by adding water-miscible solvents such as acetone.

[0089] The process of the invention permits the preparation of monodisperse fluorine-containing polymer particles having a particle diameter of 1 to 100 um, a particle diameter distribution such that the standard deviation is not more than 10% from the average particle diameter, and a fluorine content of not more than 3 wt.%.

[0090] In the process of the invention, as the polymerizable monomer, a monomer mixture of a monomer having a water solubility of 0.001 to 0.1 g/100 g H₂0 (first monomer) and having a water solubility of more than 0.1 g/100 H₂0 (second monomer) can be used. In this case, the first monomer is finely dispersed in an aqueous medium so as to satisfy the above-described requirements and then combined with a seed particle dispersion to thereby make the first monomer absorbed or adsorbed on seed particles. Subsequently the second monomer is added thereto as such, preferably after finely dispersing in an aqueous medium and more preferably so as to satisfy the above-described requirements. Upon polymerization of the monomer mixture, the described polymer particles can be prepared.

[0091] Examples of the first monomer are aromatic vinyl compounds such as styrene, a-methylstyrene, p-methylstyrene, and divinylbenzene; ethylenically unsaturated carboxylic acid alkyl esters such as butyl acrylate, butyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, lauryl acrylate, lauryl methacrylate and trimethylolpropane trimethacrylate; and conjugated diolefins such as butadiene and isoprene. These monomers can be used alone or in combination.

[0092] Examples of the second monomer are vinylpyridine, vinylidene chloride, vinyl acetate, acrylonitrile, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate, acrylamide, methacrylamide, glycidyl acrylate, glycidyl methacrylate, N-methylolacrylamide, N-methylolmethacrylamide, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, diallyl phthalate, allyl acrylate, allyl methacrylate, acrylic acid, methacrylic acid, itaconic acid and fumaric acid.

[0093] To prevent formation of new particles in the aqueous phase during the polymerization process, it is preferred that a monomer having a high rate of polymerization, such as divinylbenzene, acrylonitrile or vinyl chloride, be used as at least a part of the monomer mixture.

[0094] The aqueous dispersion of the first monomer is combined with the seed particle dispersion and gently stirred usually over more than 1 hour until the first monomer droplets are absorbed or adsorbed on the seed particles. The polymerization initiator may be absorbed or adsorbed on the seed particles simultaneously with the first monomer, but preferably it is absorbed or adsorbed on the seed particles prior to the introduction of the first monomer. In this way, swollen particles with the first monomer and the polymerization initiator absorbed or adsorbed thereon are prepared.

[0095] Then the dispersion of the second monomer is added so that the second monomer is absorbed or adsorbed on the above swollen particles.

[0096] Thereafter the temperture of the system is raised to cause polymerization. The polymerization temperature is usually 40 to 90°C. preferably 50 to 90°C although this will vary depending on the type of the polymerization initiator.

[0097] In the process of the invention, particles may be formed and grow in the aqueous phass, regardless of the seed particles, depending on the monomer composition during the polymerization process. In order to prevent the formation of new particles separately from the seed particles, known water-soluble polymerization inhibitors such as hydroxylamine, ferric chloride, potassium dichromate, and sodium sulphite can be added at the time of polymerization.

[0098] To increase the dispersion stability of the swollen particles or polymer particles, it is preferred to use a protective colloid. Typical examples of protective colloids are water-soluble polymers such as polyvinyl alcohol, polyvinyl pyrrolidone and methyl cellulose. This protective colloid is sufficient to be added in a commonly used amount.

[0099] It is generally necessary to use a dispersion stabilizer to increase the stability of dispersed particles during the polymerization process. As dispersion stabilizers, there can be used, for example, anionic and nonionic surface active agents, and organic and inorganic suspension protective agents. When, however, a surface active agent is used, it is generally necessary to control the concentration of the surface active agent to below the critical micelle concentration. A preferred example of dispersion stabilizer is polyvinyl alcohol having a degree of saponification of 75 to 95% and a degree of polymerization of 500 to 3,000.

[0100] Polymer particles prepared by the process of the invention have many advantages. One of the advantages is that the polymer particles do not contain gigantic or extremely small particles; they are highly monodisperse. Another advantage is that the polymer particles are not deformed but in a spherical form because a swelling aid is not used in preparation thereof.

[0101] The polymer particles prepared by the process of the invention, which have advantages as described above, can find many applications. They are useful as, e.g. standard samples for microscopic examination; model materials for the investigation of, e.g. separation, fluid flow, centrifugal separation, rate of diffusion, and dust; carriers for medicines for diagnosis of the living body; carrier for immobilized enzymes; powder inks; toners for electrostatic development; paints; powdered lubricants; microcapsule; spacer materials for the protection of microcapsules for pressure-sensitive copying paper; spacers for liquid crystal cells; plastic pigments for coated paper; plastic pigments for adhesives, binders for ceramics; base polymer particles for impact resistant resins; plastic pigments for cosmetics; and column fillers for ion chromatography.

[0102] In order that the invention may be well understood the following examples are given by way of illustration only. In the examples all parts and percentages are by weight unless otherwise stated.

EXAMPLE 1

[0103] Styrene (water solubility:

[0104] These ingredients were mixed with a stirrer and then finely dispersed'for 30 minutes by the use of a 300 W supersonic dispersing machine to prepare a styrene dispersion in which the maximum particle diameter of styrene droplets was 0.4 um. The particle diameter was measured with a dynamic light-scattering analyzer (Model N4 manufactured by Callter Co.).

[0105] The above dispersion was added to 300 parts of water. While gently stirring the mixture at ordinary temperature, optical micrographs were taken from time to time. Using these micrographs, changes in number average particle diameter of the styrene droplets were measured. The results are shown in Figure 1.

[0106] As can be seen from the graph of Figure 1, the number average particle diameter on standing for 3 hours under the above conditions, Dm, was 3.2 um.

[0107] Using the same ingredients as used above, a styrene dispersion was prepared in the same manner as above. 303.5 parts of the dispersion (styrene content: 100 parts: maximum particle diameter: 0.4 µm) was mixed with 200 parts of water containing a monodisperse polystyrene dispersion (polystyrene content: 7 parts: number average particle diameter: 0.40 µm) which had been prepared by soap-free polymerization. The mixture was slowly stirred for 3 hours to bring styrene droplets into contact with seed particles.

[0108] Then 100 parts of a 10% aqueous solution of polyvinyl alcohol (Gosenol GH 20 produced by Nippon Gosei Kagaku Co., Ltd.) was added to the above mixture. The system was heated to 80°C, and upon polymerization of the monomer at 80°C for 8 hours, monodisperse polymer particles having a number average particle diameter of 1.0 µm and a standard deviation of 10% in the particle diameter distribution were obtained.

[0109] The above results are shown in Table 1.

EXAMPLES 2 TO 5, and

COMPARATIVE EXAMPLES 1 TO 3

[0110] The procedure of Example 1 was repeated wherein the amount (as solids) of polystyrene particles being used as seed particles, the desired number average particle diameter D, and the contact time between styrene droplets and seed particles were changed as shown in Table 1.

[0111] The polymer particles thus prepared were measured for a number average particle diameter and the standard deviation in the particle diameter distribution. The results are shown in Table 1.

[0112] As can be seen from the results of Table 1, mono- disperse polymer particles could be obtained in Examples 1 to 5 while on the other hand, in Comparative Examples 1 to 3, only polymer particles having a broad particle diameter distribution were obtained; no monodisperse polymer particles could be obtained.

[0113] In Comparative Example 1, at the stage at which the contact time between monomer droplets and seed particles is 20 minutes, swollen particles having a nearly uniform particle diameter were obtained. However, on standing them for 3 hours, diffusion and precipitation of styrene between the swollen particles occurred, thereby producing swollen particles having a broad particle diameter distribution in a range of 0.6 to 1.5 µm. Upon polymerization of the monomer in that condition, polymer particles having a broad particle diameter distribution in a range of 0.5 to 2.0 µm were obtained.

[0114] When the monomer was polymerized at the stage that the contact time between styrene droplets and seed particles was 20 minutes, the particle diameter distribution was broadened, and polymer particles obtained also had a broad particle diameter distribution in the range of 0.6 to 2.0 µm.

[0115] In Comparative Examples 2 and 3, at the stage that the contact time between styrene droplets and seed particles was 3 hours, a large amount of styrene droplets remained unabsorbed on the seed particles. These styrene droplets did not disappear even when the system was slowly stirred for two days and nights. Some styrene droplets remaining unabsorbed became larger in diameter than those absorbed on the seed particles and even when allowed to stand for one week, they were not absorbed on the seed particles. Upon polymerization of the monomer in that condition, polymer particles having a broad particle diameter distribution in a range of 5 to 20 µm in Comparative Example 2 and in a range of 5 to 30 pm in Com- ^parative Example 3 were obtained.

[0116] EXAMPLES 6 TO 8. AND

[0117] COMPARATIVE EXAMPLES 4 AND 5

[0118] 

[0119] The above ingredients were stirred in the same manner as in Example 1 to prepare a dispersion in which MMA was finely dispersed (maximum particle diameter, 0.7 µm). The number average particle diameter in a semi-stable condition, Dm, was 26 pm.

[0120] The procedure of Example 1 was repeated wherein the same MMA dispersion as prepared above (maximum particle diameter: 0.7 µm) was used as a monomer dispersion, a monodisperse polystyrene latex (number average particle diameter: 0.82 µm) prepared by soap-free polymerization was used as seed particle, and the amount (solids) of the seed particles used, the desired number average particle diameter D and the contact time between the the MMA droplets and seed particles were changed as shown in Table 2.

[0121] The polymer particles thus prepared were measured for the number average particle diameter and standard deviation in the particle diameter distribution. The results are shown in Table 2.

[0122] As can be seen from the results of Table 2, mono- disperse polymer particles could be obtained in Examples 6 to 8 while on the other hand in Comparative Examples 4 and 5, polymer particles had a broad particle diameter distribution; no monodisperse polymer particles could be obtained.

[0123] In Comparative Examples 4 and 5, the particle diameters of swollen particles were not uniform at the stage that the contact time between styrene droplets and seed particles was 3 hours. The polymer particles obtained by polymerization in that condition had broad particle diameter distributions ranging between 2 and 20 µm, and between 3 and 50 µm.

EXAMPLE 9, AND

COMPARATIVE EXAMPLE 6

[0124] 2-Ethylhexyl acrylate (water solubility:

[0125] 

[0126] The above ingredients were stirred in the same manner as in Example 1 to prepare a dispersion in which 2-ethylhexyl acrylate was finely dispersed (maximum particle diameters 0.09 pm). After allowing to stand for 3 hours, the above dispersion was measured for the number average particle diameter in a semi-stable condition, Dm, by the use of a dynamic light-scattering analyzer (Model N4 manufactured by Callter Co.). Dm was 0.32 µm.

[0127] The same 2-ethylhexyl acrylate dispersion as prepared above was used as a monomer dispersion, and mono- disperse polymethyl methacrylate particles (number average particle diameters 0.12 µm) were used as seed particles. The amount (solids) of polymethyl methacrylate particles used, the desired number average particle diameter D, and the contact time between 2-ethylhexyl acrylate droplets and seed particles were determined as shown in Table 3.

[0128] The polymethyl methacrylate particle dispersion and the seed particle dispersion were combined together, and then the system was heated to 75°C at which the monomer was polymerized.

[0129] The polymer particles thus obtained were measured for the number average particle diameter and deviation in the particle diameter distribution. The results are shown in Table 3.

[0130] As can be seen from The results of Table 3, mono- disperse polymer particles could be obtained in Example 9 while on the other hand, in Comparative Example 6, the polymer particles obtained had a broad particle diameter distribution; polymer particles having a uniform particle distribution could not be obtained.

[0131] In Comparative Example 6, a mixture of uniform swollen particles having a diameter of 0.8 µm and monomer droplets having a broad particle diameter distribution was obtained at the stage that the contact time between monomer droplets and seed particles was 3 hours. Even though the mixture was allowed to stand for one week, the monomer droplets did not disappear. Upon polymerization of the monomer while maintaining the temperature of the system at 75°C in that condition, only polymer particles having a broad particle diameter distribution in a range of 0.1 to 0.9 µm were obtained.

EXAMPLE 10. AND

COMPARATIVE EXAMPLE 7

[0132] In this example, swollen particles having a uniform particle diameter of more than 10µm were prepared using styrene.

[0133] These ingredients were dispersed with supersonic waves to prepare a fine dispersion (diameter: less than 0.5 µm).

[0134] This dispersion was combined with a mixture of 200 parts of water containing 5 parts of the same polyvinyl alcohol as used in Example 1 and 50 parts of methanol, and then gently stirred. The number average particle diameter of monomer droplets after 3 hours, i.e., Dm was measured with an optical microscope. Dm was 22.5 pm.

[0135] The same styrene dispersion as prepared above was used as a monomer dispersion. Monodisperse polystyrene particles having a number average particle diameter of 0.82 µm were used as seed particles. The amount (solids) of the polystyrene particles used, the desired number average particle diameter D, and the contact time between styrene droplets and seed particles were determined as shown in Table 4. After combining the styrene dispersion and the seed particle dispersion, the temperature of the system was raised to 80°C, and polymerization was carried out at 80°C.

[0136] The amount of the styrene droplets used was 303.5 parts, and the monodisperse polystyrene particle dispersion was prepared by mixing the polystyrene particles, 5 parts of polyvinyl alcohol, 200 parts of water and 50 parts of methanol.

[0137] The polymer particles thus prepared were measured for a number average particle diameter and a deviation in the particle diameter distribution. The results are shown in Table 4.

[0138] As can be seen from the results of Table 4, mono- disperse polymer particles could be obtained in Example 10 while on the other hand, in Comparative Example 7, only polymer particles having a broad particle diameter distribution were prepared, no monodisperse polymer particle could be obtained.

[0139] In Comparative Example 7, swollen particles having a broad particle diameter distribution in a range of 4 to 7 pm were formed at the stage that the contact time was 1 hour. At the stage that the contact time was 3 hours, swollen particles had a broad particle diameter in a range of 3 to 20 µm. Upon polymerization of the monomer in that condition, only polymer particles having a broad particle diameter distribution in a range of 3 to 30 pm were obtained.

EXAMPLE 11

[0140] 303.5 parts of the same styrene dispersion as prepared in Example 10 was mixed with a mixture of 2.0 parts of the monodisperse . polystyrene particles having a number average particle diameter of 11.6 µm, 5 parts of polyvinyl alcohol, 200 parts of water and 50 parts of methanol. On slowly stirring the resulting mixture for 12 hours, monodisperse swollen particles having a number average particle diameter of about 40 µm were obtained.

[0141] 50 parts of a 10% aqueous solution of polyvinyl alcohol and 5 parts of a 10% aqueous solution of ferric chloride were futher added. Upon polymerization of the monomer at 70°C for 8 hours, monodisperse polystyrene particles having a number average particle diameter of 42 µm and a deviation value of 11% were obtained in a polymerization yield of 97%.

[0142] In the above styrene dispersion, the number average particle diameter in a semi-stable condition, Dm,. was 22.5 µm, the desired number average particle diameter D was 42.0 µm, and Dm/D was 0.54.

EXAMPLE 12

[0143] In this example, polymethyl methacrylate particles having a number average particle diameter smaller than that of the usual particles were prepared. MMA 100 parts Isoparaffin (Shellsol 71, produced by Shell Petroleum Co., Ltd., water solubilityi

[0144] 

[0145] These ingredients were mixed, and then added to a mixture of 200 parts of water and 2 parts of sodium laurylsulfate. The resulting mixture was finely dispersed with supersonic waves to prepare a MMA dispersion (maximum particle diameter, less than 0.5 µm).

[0146] Dm of the above MMA dispersion as determined in the same manner as in Example 1 was 2.1 µm. The desired number average particle diameter D was 2.5 µm and Dm/D was 0.84.

[0147] The above MMA dispersion was again prepared, and added to a mixture of 1.4 parts of monodisperse polystyrene particles (number average particle diameter, 0.60 µm), 5 parts of polyvinyl alcohol, and 300 parts of water. On stirring the resulting mixture for 3 hours, swollen particles having a uniform particle diameter were obtained. It was then added to 50 parts of a 10% aqueous solution of polyvinyl alcohol. Upon polymerization of the monomer at 70°C for 5 hours, uniform polymer particles having a number average particle diameter of 2.5 pm and a standard deviation of 7% were obtained.

EXAMPLE 13

[0148] In this example, polymer particles having a larger particle diameter than the usual one were prepared using 2-ethylhexyl acrylate.

Benzoyl peroxide (dissolved in 2-ethylhexyl 2 parts acrylate)

[0149] These ingredients were finely dispersed by the use of a supersonic dispersing machine to prepare a 2-ethylhexyl acrylate dispersion (maximum particle diameters less than 0.2 µm).

[0150] The above 2-ethylhexyl acrylate dispersion was added to a mixture of 7 parts of polyvinyl alcohol, 3 parts of calcium chloride and 200 parts of water, and slowly stirred. After allowing to stand for 3 hours in the same manner as in Example 1, the number average particle diameter, i.e., Dm, was measured. Dm was 0.8 µm. In this example, the desired number average particle diameter D is 0.92 µm and Dm/D is 0.87.

[0151] The above dispersion was again prepared, and then added to a mixture of 1 part of a monodisperse styrene-butadiene latex (number average particle diameter, 0.20 µm), 7 parts of polyvinyl alcohol, 3 parts of calcium chloride and 200 parts of water. On slowly stirring the resulting mixture for 3 hours, a dispersion of mono- disperse swollen particles having a number average particle diameter of about 1 pm was obtained.

[0152] The temperature of the mixture was raised to 70°C. Upon polymerization of the monomer for 5 hours, uniform polymer particles having a number average particle diameter of 0.92 µm and a standard deviation of 6% were obtained.

EXAMPLE 14

[0153] 303 parts of the same styrene dispersion as in Example 6 was added to a mixture of a dispersion of 1.0 part of polystyrene particles having a number average particle diameter as obtained in Example 11, 5 parts of a 10% aqueous solution of polyvinyl alcohol, 20 parts of a 1% aqueous solution of ferric chloride, 200 parts of water and 70 parts of acetone. On slowing stirring the resulting mixture at 10°C for 48 hours, monodisperse swollen particles having a number average particle diameter of about 200 pm were obtained.

[0154] Upon polymerization of the monomer at 70°C for 8 hours, monodisperse polymethyl methacrylate particles having a number average particle diameter of 186 µm and a standard deviation of 7% were obtained.

[0155] In the above styrene dispersion, the number average particle diameter in a semi-stable condition, Dm, is 150 µm, the desired number average particle diameter is 194 µm, and Dm/D is 0.77.

EXAMPLE 15

[0156] 

[0157] These ingredients were stirred with supersonic waves to prepare a dispersion of Perbutyl 0 (maximum particle diameter: 0.5 µm; Dm: 1.0 µm). This dispersion was added to 40 parts of a monodisperse polystyrene latex (number average particle diameter: 0.80 µm; solids contenti 5%). The mixture was slowly stirred at 30°C over 12 hours to thereby make droplets of Perbutyl 0 absorbed on polystyrene particles as seed particles.

[0158] These ingredients were finely dispersed with supersonic waves to prepare a dispersion in which a particle diameter of droplets of the styrene-divinylbenzene mixture was less than 0.5 µm. This dispersion was added to the above seed,particle dispersion. By slowing stirring the resulting mixture at 30°C for 12 hours, the monomer mixture was made absorbed on the seed particles.

[0159] Thereafter, 100 parts of a 10% aqueous solution of polyvinyl alcohol (Gosenol GH20 produced by Nippon Gosei Kagaku Co., Ltd.) was added. The temperature was raised to 80°C to start polymerization. Polymerization was almost completed in 4 hours. Almost no aggregation occurred and the formation of new particles in an aqueous layer was not observed at all.

[0160] The polymer particles thus obtained were examined with a scanning-type electron microscope. It was confirmed that the particles were nearly spherical in shape, the number average particle diameter was 2.8 pm, and the deviation of particle diameter was 5%; the particles had a greatly uniform particle diameter.

[0161] A scanning-type electron micrograph of the polymer particles is shown in Fig. 2 (x 1200).

REFERENCE EXAMPLE 1

[0162] 

[0163] These ingredients were finely dispersed with supersonic waves to such an extent that the maximum particle diameter of the resulting oil droplets was less than 0.5 µm. During this process, the temperature of the dispersion rised to 60°C as a result of heat generation due to the dispersion operation.

[0164] The dispersion thus prepared was added to 2 parts (calculated as solids) of a monodisperse polystyrene latex (solids content: 5%) having a number avarage particle diameter of 0.80 µm. The resulting mixture was slowly stirred at 30°C over 24 hours to make the monomer and polymerization initiator droplets absorbed on the seed particles. Even at the end of the period, the droplets did not disappear. Even though the mixture was stirred for additional 48 hours, the droplets did not disappear.

[0165] Thereafter, 100 parts of a 10% aqueous solution of polyvinyl alcohol was added to the above mixture. The temperature of the system was raised to 80°C, and upon polymerization of the monomer at that temperature for 4 hours, polymerization was nearly completed.

[0166] The polymer particles thus obtained were examined with a scanning-type electron microscope. It was found that the polymer particles were composed of particles having a uniform particle diameter of about 2.7 µm and particles having an uneven particle diameter ranging from 0.5 to 2 pm.

[0167] A scanning-type electron micrograph of the polymer particles is shown in Fig. 3 (x 1200).

EXAMPLE 16

[0168] 

[0169] These ingredients were finely dispersed with supersonic waves to such an extent that the maximum particle diameter of droplets was less than 0.6 µm. The dispersion thus prepared was added to 50 parts of an aqueous dispersion (solids content: 2%) of polystyrene particles having a number average particle diameter of 0.9 µm. The resulting mixture was slowly stirred at 30°C for 24 hours to thereby make Perloyl 355 absorbed on the polystyrene particles as seed particles.

[0170] 

[0171] These ingredients were finely dispersed with a Mantongaurin homogenizer to prepare a styrene dispersion (number average particle diameteri less than 0.8 µm; Dm: 3.2 pm). This styrene dispersion was added to the above seed particle dispersion, and the resulting mixture was slowly stirred at 30°C for 10 hours to make the styrene absorbed on the seed particles. In this way, swollen particles having a uniform particle diameter of 6.0 µm were prepared.

[0172] These ingredients were finely dispersed with a Mantongaurin homogenizer to prepare a butyl aerylate dispersion (number average particle diameters less than 0.8 µm; Dm: 9 µm). This dispersion was added to the above seed particles, and the resulting mixture was slowly stirred at 30°C for 5 hours to thereby make the butyl acrylate absorbed on the seed particles.

[0173] These ingredients were finely dispersed with a Mantongaurin homogenizer to prepare an acrylonitrile dispersion (number average particle diameter: less than 0.5 µm; Dm:13 pm). This acrylonitrile dispersion was added to the above dispersion, and the resulting mixture was slowly stirred at 30°C for 42 hours to thereby make the acrylonitrile absorbed on the seed particles.

[0174] The thus-prepared swollen seed particles had a uniform particle diameter of about 9 µm. To the dispersion thus obtained was added 1,000 parts of a 10% aqueous solution of polyvinyl alcohol, and then the resulting mixture was heated to 70°C to start polymerization. Polymerization was nearly completed in 6 hours. Optical microscopic examination confirmed that the polymer particles were nearly spherical particles having a number average particle diameter of 8.6 µm and a standard deviation of 7%.

EXAMPLE 17

[0175] 

[0176] These ingredients were finely dispersed with a Mantongaurin homogenizer to prepare a dispersion in which the particle diameter of droplets was less than 0.5 µm. This dispersion was immediately added to 50 parts of a monodisperse polystyrene latex (number average particle diameters 0.83 µm), and the resulting mixture was slowly stirred at 20°C for 24 hours to make the droplets of Perbutyl 0 absorbed on the seed particles.

[0177] These ingredients were finely dispersed with a Mantongaurin homogenizer to prepare a styrene dispersion in which the maximum particle diameter of droplets of styrene was less than 0.5 µm. This dispersion was divided into two portion. Each portion was added over 12 hours to make the styrene droplets absorbed on the seed particles.

[0178] These ingredients were finely dispersed with a Mantongaurin homogenizer to prepare a dispersion in the maximum particle diameter of acrylonitrile/1-chlorododecane droplets was less than 0.7 pm.

[0179] This dispersion was divided into four portions. Each portion was added to the above seed particles to make the monomer absorbed on the seed particles.

[0180] Optical microscopic examination showed that oil droplets comprising swollen seed particles had a uniform particle diameter of about 9 pm.

[0181] To the above-prepared dispersion were added 1,500 parts of a 10% aqueous solution of polyvinyl alcohol (Gosenol GH20 produced by Nippon Gosei Kagaku Co., Ltd.) and 5 parts of a 10% aqueous solution of ferric chloride. Upon polymerization at 80°C for 10 hours, polymer particles having a uniform particle diameter and spherical in shape (number average particle diameter: 9.2 µm; standard deviation in the particle diameter distribution: 3%) were obtained.

EXAMPLE 18

[0182] 10 parts of magnetite (EPT produced by Toda Kogyo Co., Ltd.) was dispersed in 60 parts of a 0.005 mol/l aqueous solution of ferric chloride (particle diameter; 1 to 5 µm), thereby providing the surface of the magnetite particles with positive charges.

[0183] 40 parts of a 10% aqueous solution of sodium oleate was added to the above dispersion to thereby make the oleic acid absorbed on the particles. The dispersion was adjusted to pH 6-7 using diluted hydrochloric acid to remove an excess of oleic acid. In this way, magnetite was made oleophilic. These magnetite particles were used as seed particles. They were dispersed in 200 parts of a 5% aqueous solution of polyvinyl alcohol (Gosenol GH20) to prepare a dispersion of seed particles having a particle diameter of 0.3 to 0.5 µm.

[0184] 

[0185] These ingredients were emulsified with a Mantongaurin homogenizer to an extent that the diameter of styrene droplets was less than 0.1 pm. This styrene dispersion was added to the above seed particle dispersion, and the resulting mixture was slowly stirred over 18 hours to make the styrene absorbed on the surface of the magnetite.

[0186] These ingredients were emulsified in such a manner that the particle diameter of droplets was less than 0.1 µm. This dispersion was added to the above dispersion to make the azobisisobutyronitrile absorbed on the surface of the magnetite.

[0187] Upon polymerization at 70°C for 10 hours, magnetite containing polymer particles having a particle diameter range of 2 to 7 µm and a magnetic substance content of 8% were obtained.

EXAMPLE 19

[0188] 

[0189] These ingredients were emulsified with supersonic waves to prepare a dispersion in which a particle diameter of Perbutyl 0 droplets was less than 0.5 µm.

[0190] To this Perbutyl 0 dispersion was added 50 parts of a monodisperse polystyrene latex (number average particle diameteri 0.80 µm; solids contents 2%), and the resulting mixture was slowly stirred at 30°C for 8 hours to make the Perbutyl 0 absorbed on the polystyrene particles as seed particles.

[0191] 

[0192] These ingredients were emulsified with supersonic waves to prepare a dispersion in which the maximum particle diameter of styrene droplets was less than 0.5 µm, and Dm was 3.2 pm. This styrene dispersion was added to the above seed particle dispersion, and the resulting mixture was slowly stirred at 30°C for 12 hours to make the styrene absorbed on the seed particles.

[0193] Then 50 parts of acrylonitrile was added to the above dispersion without finely dispersing it, and the resulting mixture was stirred for one hour.

[0194] Thereafter 200 parts of a 5% aqueous solution of polyvinyl alcohol (Gosenol GH produced by Nippon Gosei Kagaku Co., Ltd.) was added. The temperature was raised to 80°C to start polymerization. The polymerization was almost completed in 6 hours. The formation of aggregate was not almost observed, and the formation of new particles in an aqueous layer was not observed at all.

[0195] The polymer particles thus prepared were examined with a scanning-type electron microscope. It was confiered that the polymer particles were nearly spherical in shape and had a greatly uniform particle diameter (number average particle-diameter: 3.5 µm; standard deviation ir the particle diameter distribution: 4%).

EXAMPLES 20 AND 21

[0196] Polymer particles were prepared in the same manner as in Example 19 except that 0.1 part of sodium laurylsulfate was used in place of the polyvinyl alcohol in Example 20 and 0.2 part of sodium larylsulfate was used in place of the polyvinyl alcohol in Example 21. In these cases, since dispersion stability during the polymerization process was poor, the system was stirred slowly and intermittently.

[0197] In Example 20, polymer particles having a uniform particle diameter (number average particle diameter: 3.2 µm; standard deviation: 10%) were obtained. The amount of aggregate formed was 4.5%.

[0198] In Example 21, polymer particles having a uniform particle diameter (number average particle diameter: 3.1 µm; standard deviations 8%) were obtained. The amount of aggregate formed was 2.8%.

EXAMPLES 22 TO 29

[0199] Polymer particles were prepared in the same manner as in Example 19 except that different polyvinyl alcohols having the properties as shown in Table 5 were used.

[0200] The number average particle diameter and standard deviation of polymer particles obtained, and the amount of aggregate formed in each example are shown in Table 5. The results of Example 19 are also shown in Table 5.

EXAMPLE 30

[0201] 

[0202] These ingredients were emulsified with a Mantongaurin homogenizer (Model 15M) to prepare a dispersion in which a particle diameter of monomer droplets was less than 0.4 µm. This operation was performed while cooling so that the temperature of the system did not exceed 25°C.

[0203] The dispersion thus prepared was added to 50 parts of a monodisperse polystyrene latex (number average particle diameters 0.7 µm; solids content: 2%), and the resulting mixture was slowly stirred at 25°C for 48 hours to make the monomer absorbed on the polystyrene particles.

[0204] 20 parts of 4-vinylpyridine was added, and moreover 100 parts of a 10% aqueous solution of polyvinyl alcohol (Gosenol GH20) was added. The temperature of the resulting mixture was raised to 70°C to start polymerization. The polymerization was nearly completed in 5 hours.

[0205] The polymer particles thus prepared were examined with an optical microscope. It was confirmed that the polymer particles were nearly spherical particles having a number average particle diameter of 8.6 µm and a standard deviation of 5%.

[0206] The polymer particles were hydrophilic on the surface thereof and thus had good dispersibility in water.

EXAMPLE 31

[0207] 2 parts of tert-butylperoxy 2-ethylhexanoate (Perbutyl 0 produced by Nippon Yushi Co., Ltd.) as a polymerization initiator, 85 parts of 2-ethylhexyl acrylate as the monomer of the general formula (I), and a mixture of 7 parts of styrene and 8 parts of divinylbenzen as the other monomer component were uniformly mixed and then finely dispersed in 200 parts of water with 1 part of sodium laurylsulfate dissolved therein with supersonic waves to an extent that the particle diameter of monomer droplets was less than 0.8 µm.

[0208] The above-prepared aqueous monomer dispersion was added to a monodisperse polystyrene latex (number average particle diameters 0.90 µm; solids content 5%), and the resulting mixture was slowly stirred at 30°C for 12 hours to make the monomer droplets absorbed on the polystyrene particles as seed particles.

[0209] The above-prepared swollen seed particles had a uniform particle diameter of about 2 µm.

[0210] 100 parts of a 10% aqueous solution of polyvinyl alcohol (Gosenol GH20 produced by Nippon Gosei Co., Ltd.) was added to the above dispersion. The temperature was raised to 80°C to start polymerization. The polymerization was nearly completed in 4 hours. The formation of agrregate was not almost formed, and the formation of new particles in an aqueous layer was not observed at all. The polymer particles thus prepared were examined with a scanning-type electron microcope. It was confirmed that the polymer particles were nearly spherical particles having a greatly uniform particle diameter (number average particle diameteri 2.1 µm; standard deviation: 5%).

[0211] This aqueous dispersion of the polymer particles was diluted with water to an extent that the solids content was 1%. 500 ml of the aqueous dispersion was placed in a 500-milliliter graduated cylinder and was subjected to a standing stability test. After 30 days, formation of a sedimentation layer was examined. It was found that sedimentation did not occur at all.

EXAMPLE 32

[0212] 

[0213] These ingredients were emulsified with supersonic waves to prepare a dispersion in which a particle diameter of droplets was less than 0.7 µm. This dispersion was added to a mixture of 10 parts of a monodisperse polystyrene latex (number average particle diameter; 0.71 µm; solids contenti 10.0%) and 7 parts of acetone. The resulting mixture was slowly stirred at 25°C for 6 hours to make the droplets on the polystyrene particles as seed particles.

[0214] A mixture of 82 parts of 2-ethylhexyl methacrylate and 18 parts of styrene was finely dispersed in 300 parts of water with 2 parts of sodium laurylsulfate with supersonic waves to an extent of a particle diameter of droplets was less than 0.7 pm. This aqueous monomer dis- persion was added to the above aqueous seed particle dispersion, and the resulting mixture was slowly stirred at 40°C for 6 hous to make the monomer absorbed on the seed particles, thereby producing swollen particles.

[0215] 100 parts of a 10% aqueous solution of polyvinyl alcohol (Gosenol GH20) was added to the above dispersion. In addition, 10 parts of a 5% aqueous solution of potassium dichromate as a polymerization inhibitor was added. The temperature was raised to 70°C, at which polymerization was carried out for 10 hours. The final conversion was 96%. The formation of aggregate and new particles was not almost observed.

[0216] The polymer particles thus prepared were examined with an optical microscope. It was found that the polymer particles were monodisperse, spherical particles having a uniform particle diameter (number average particle diameter: 3.3 µm; standard deviation: 3.6%).

[0217] The aqueous dispersion containing the polymer particles was diluted with water so that the solids content was 1%. This aqueous dispersion was placed in a 500-milliliter graduated cylinder and was subjected to a 30-day standing stability test. Sedimentation did not occur at all.

EXAMPLE 33

[0218] The procedure of Example 32 was repeated wherein 5 parts of the polymer latex prepared in Example 32 (particle diameter: 3.3 µm; solids concentrations 20.0%) was used in place of 10 parts of the monodisperse polystyrene latex (particle diameters 0.71 µm; solids contents 10.0%), and as the monomer, lauryl methacrylate was used in place of 2-ethylhexyl methacrylate. A mono- disperse polymer particle dispersion (number average particle diameters 15.1 µm) was obtained.

[0219] This dispersion was diluted with water so that the solids content was 1%. 500 ml of the dispersion was placed in a 500-milliliter graduated cylinder and was subjected to a standing stability test. After 30 days, the dispersion was examined. Sedimentation did not occur at all, and an upper aqueous layer was uniformly turbid.

EXAMPLE 34

[0220] 

[0221] These ingredients were emulsified with a supersonic homogenizer in such a manner that the maximum particle diameter of droplets was less than 0.5 µm.

[0222] The above dispersion was added to a mixture of 5 parts of a monodisperse polystyrene latex (particle diameteri 0.7 µm; solids content: 10%) and 6 parts of acetone, and the resulting mixture was slowly stirred at 25°C for 12 hours to make the droplets on the polystyrene particles as seed particles.

[0223] These ingredients were emulsified with a high-pressure piston-type homogenizer (Type 15 M manufactured by Mantongaurin Co.) to prepare a dispersion in which the maximum particle diameter of droplets was less than 0.5 µm.

[0224] This dispersion was added to the above seed particle dispersion. The resulting mixture was slowly stirred at 25°C for 1 hour to make the monomer absorbed on the seed particles.

[0225] 200 parts of a 5% aqueous solution of polyvinyl alcohol was added. Polymerization was carried out at 70°0 for 6 hours. Fluorine-containing polymer particles having a uniform particle diameter (number average particle diameter: 4.1 µm; standard deviation: 3%) were obtained in a yield of 98%.

[0226] A scanning-type electron micrograph (x2000) of the polymer particles is shown in Fig. 4, and an infrared absorption spectrum (by the KBr tablet method) of the polymer particles is shown in Fig. 5. The fluorine content (as determined by the elemental analysis method) of the polymer was 42.3 wt%.

[0227] In order to examine non-aggregation properties of the polymer particles, the following test was carried out.

[0228] The polymer particles were washed with water, dried and powdered. About 10 mg of the powder was placed on one end of a glass plate (20 cm x 20 cm). On blowing the powder using a blower provided with a brush for claaning camera lenses, the powder was scattered in a mist form and uniformly extended over the top and back sides of the glass plate.

[0229] Examination of the powder on the galss plate with an optical microscope showed that the polymer particles were uniformly scattered and existed nearly one by one.

EXAMPLE 35

[0230] The procedure of Example 34 was repeated wherein 5 parts of a monodisperse polystyrene latex (number average particle diameter: 0.20 µm; solids content: 10%) was used as the seed polymer latex. Fluorine-containing polymer particles having a uniform particle diameter (number average particle diameteri 1.2 µm; standard deviations 4%) were obtained in a yield of 98%.

[0231] The fluorine-content of the polymer particles as determined by the elemental analysis method was 42.2 wt%.

EXAMPLE 36

[0232] The procedure of Example 34 was repeated wherein as the monomers, 30 parts of 2,2,2-trifluoroethyl methacrylate (Viscoat 3FM produced by Osaka Yuki Kagaku Kogyo Co., Ltd.), 5 parts of divinylbenzene, 5 parts of styrene, and 60 parts of methyl meethacrylate were used. Polymer particles were prepared in a yield of 96%.

[0233] These polymer particles were fluorine-containing polymer particles having a uniform particle diameter (number average particle diameters 4.0 µm; standard deviation: 3.5%). The fluorine content (as determined by the elemental analysis method) of the polymer was 10.2%.

EXAMPLE 37

[0234] The procedure of Example 34 was repeated wherein as the monomers, 15 parts of 2,2,2-trifluoroethyl methacrylate (Viscoat 3FM produced by Osaka Yuki Kagaku Kogyo Co., Ltd.), 5 parts of divinylbenzene, 5 parts of styrene, and 75 parts of methyl methacrylate were used. Polymer particles were obtained in a yield of 95%. These polymer particles were fluorine-containing polymer particles having a uniform particle diameter (number average particle diameter: 4.0 µm; standard deviation: 3.1%). The fluorine content (as determined by the elemental analysis method) of the polymer was 4.9%.

Claims

1. A process for preparing large-sized polymer particles having a particle diameter in a range of 0.1 to 500 µm by (a) dispersing a polymerizable monomer in an aqueous medium to prepare a monomer dispersion, (b) combining said monomer dispersion with a dispersion of seed particles to that the polymerizable monomer is absorbed or adsorbed on the seed particles, and (c) polymerizing the polymerizable monomer in the presence of a polymerization initiator; characterized in that (i) the number average particle diameter of the monomer droplets in the polymer dispersion is not larger than the number average particle diameter of the seed particles and (ii) that the number average particle diameter in a semi-stable condition, Dm, of the monomer droplets in the monomer dispersion satisfies the relationship:

in which D is the desired number average particle diameter of the final polymer.

2. A process as claimed in Claim 1, characterized in that the number average particle diameter in a semi-stable condition, Dm, of the monomer droplets in the dispersion satisfies the relationship:

(in which D is as defined in claim 1).

3. A process as claimed in Claim 1 or Claim 2 characterized in that the maximum particle diameter of the monomer droplets of the monomer dispersion is not larger than the number average particle diameter of the seed polymer particles.

4. A process as claimed in any one of the preceding claims characterized in that the number average particle diameter in a semi-stable condition, Dm, of the monomer droplets in the monomer dispersion is controlled by the addition of an oily substance having a water solubility of not more than 1/100 of that of the polymerizable monomer.

5. The process as claimed in any one of claims 1 - 3 characterized in that the number average particle diameter in a semi-stable condition, Dm, of the monomer droplets in the monomer dispersion is controlled by the addition of an organic solvent which completely miscible with water, or an inorganic metal salt.

6. A process as claimed in any one of the preceding claims characterized in that the polymerization initiator is an oil-soluble polymerization initiator.

7. A process as claimed in any one of the preceding claims characterized in that the polymerization initiator is finely dispersed in an aqueous medium and combined with the dispersion of seed particles independently of the polymerizable monomer.

8. A process as claimed in Claim 7 characterized in that the dispersion of the polymerization initiator is combined with the dispersion of seed particles prior to the introduction of the polymerizable monomer dispersion.

9. A process as claimed in any one of the preceding claims characterized in that the polymerizable monomer is a monomer mixture containing 40 to 100 wt% of a monomer of the formula:

in which R is a hydrogen atom or a methyl group, and _R ² is a C₂-^C₁₈ alkyl group.

10. A process as claimed in any one of claims 1 - 8 characterized in that the polymerizable monomer is a monomer mixture containing at least 10 wt.% of a fluorine-containing polymerizable monomer having a fluorine content of not less than 25 wt.%.

11. A process as claimed in Claim 10 characterized in that before combining the polymerizable monomer dispersion with the dispersion of seed particles, an oleophilic substance is adsorbed on the seed particles.

12. A process as claimed in any one of claims 1 - 8 characterized in that the polymerizable monomer is a mixture of a polymerizable monomer having a water solubility of 0.001 g/100 g H₂0 (first monomer) and a polymerizable monomer having a water solubility of not less than 0.1 g/100 g H₂O (second monomer).

13. A process as claimed in Claim 12 characterized in that firstly, the first monomer is finely dispersed in an aqueous medium and then absorbed or adsorbed on the seed particles; and then the second monomer is finely dispersed and the absorbed or adsorbed on the seed particles.

14. The process as claimed in any one of the preceding claims characterized in that the desired number average particle diameter, D (um), is determined according to the equation:

wherein Ds is an average particle diameter (µm) of the seed particles, Mm is an amount (g) of the polymerizable monomer used, and Ms is the amount (calculated as solids) (g) of the seed particles.

15. A process as claimed in any one of the preceding claims characterized in that the polymerization is carried out in the presence of a dispersion stabilizer comprising a suspension protective agent and/or, at a concentration below the critical micelle concentration, a surface active agent.

Drawing